[Rhodes22-list] Re: Pointing

[brad haslett]

Boys, Boys, Boys!

You guys are starting to scare me!  Bernoulli?  Pizza
maker, right?  Newton makes a damn fine fig bar!  I'd
be willing to bet that the sailors on this list who
race, Anne for example, give little thought to
aerodynamic theory in the heat of battle.  Slim, do
you think about music theory in the middle of a good
lick?  Competence in any field comes from experience
and practice.

You guys must drink less when you sail than I do!

Brad Haslett
"CoraShen" 
--- Steve Alm <salm at mn.rr.com> wrote:

> Roger and Peter,
> 
> A musician should know better than to talk physics
> -- especially around
> here.  I'll shut up now.  8-)
> 
> Slim
> 
> On 9/23/04 8:25 AM, "Roger Pihlaja"
> <cen09402 at centurytel.net> wrote:
> 
> > Slim,
> > 
> > Actually, all of the foils can stall out, both in
> the water & in the air.
> > An object does not have to be a certain shape to
> generate lift.  To prove
> > this to yourself, stick your hand out of the car
> window.  If you hold your
> > hand at an angle to the air flow, do you feel a
> force?  That's lift!  Is
> > your hand shaped like an airfoil?  Even a flat
> plate can generate lift if it
> > is held at an angle of attack to the fluid flow. 
> The fluid does not have to
> > be a gas, like air, either.  It turns out liquids
> obey the same laws of
> > hydrodynamics as gases.  The only differences
> between gases and liquids show
> > up in the defining equations as terms for density
> & viscosity.  Liquids are
> > usually more dense and more viscous than gases as
> the same temperature &
> > pressure.  Without going into the physics, what
> this means is that dense
> > liquids will produce the same amount of lift
> force/unit area at a lower
> > fluid velocity than gases.  Or alternatively, at
> the same fluid velocity,
> > liquids require less surface area to produce a
> given amount of lift force.
> > For example, at room temperature & pressure, the
> density of air is about
> > 0.076 lb/ft^3 vs. water at about 62.4 lbs/ft^3, a
> factor of about 800X.  So,
> > the keel only needs to have about 1/800 the
> surface area of the sails to
> > generate the lift forces required to resist leeway
> under sail.  Water is
> > also much more viscous than air.  This has the
> effect of making the
> > underwater foils much more forgiving or less prone
> to stalling out than the
> > sails.  This is a good thing because it makes
> sailing much easier.  If your
> > underwater foils stalled out as easily as your
> sails; then, every time the
> > boat lifted in a wave or every time you moved the
> rudder blade, these foils
> > would stall out & quit generating lift.  However,
> at a sufficiently high
> > angle of attack, even your underwater foils will
> stall out & quit generating
> > lift.  This happens most frequently with the
> rudder blade.  If we define the
> > angle between the tiller & the centerline of the
> boat as the angle of attack
> > of the rudder blade; then, the rudder blade is
> starting to stall out at an
> > angle of about 30 degrees & completely stalled out
> at an angle of about 45
> > degrees.  At angles greater than about 45 degrees,
> the rudder blade behaves
> > more like a water brake or drag device than an
> underwater foil.  So, unless
> > you are trying to slow down the boat, putting the
> tiller over more than
> > about 45 degrees off the centerline is
> counterproductive as far as steering
> > goes.
> > 
> > People cite the analogy of airflow moving faster
> over the curved surface of
> > the top of a wing vs., the straight bottom surface
> as causing a pressure
> > difference between the top & bottom surfaces &
> that's what causes lift.  In
> > the middle 1700's, a Swiss mathematician &
> scientist named Bernoulli Bioplus
> > did a mass & energy balance on all the forms of
> energy contained within a
> > moving fluid.  These days, mass & energy balances
> are fundamental to
> > engineering calculations.  But, in Bernoulli's
> time, this was a completely
> > new & creative approach!  Bernoulli found that, if
> you keep a running tally
> > on all the forms of energy in the fluid as it
> flows from place to place;
> > then, total energy is conserved.  The energy can
> change form - i.e. kinetic
> > energy can be traded off for pressure &/or
> potential energy & vice versa;
> > but, the total amount of energy remains constant. 
> Bernoulli expressed this
> > idea in the form of an equation that now bears his
> name.  Bernoulli's
> > equation is one of the 1st things students learn
> in any class on fluid flow
> > or hydrodynamics.  Naval architects, aeronautical
> engineers, & chemical
> > engineers have it tattooed on the inside of their
> eyelids so they see it in
> > their sleep!  Macroscopically, one of the things
> Bernoulli's equation
> > predicts & experimental measurements have verified
> is that there is a high
> > pressure region on the windward side of a sail, a
> low pressure region on the
> > leeward side of a sail, & greater air velocity on
> the leeward side vs. the
> > windward side - hence the common analogy cited
> above.  The difference
> > between these two air pressures, multiplied by the
> surface area of the
> > sailcloth over which the pressure difference is
> acting, is a force, which we
> > call "lift".  Although Bernoulli's equation is
> correct, it doesn't provide
> > much insight into what's actually going on,
> physically.  Physically, what's
> > actually happening is Newton's Laws of Motion are
> at work, as always.  The
> > air flowing over the sail is being forced to
> change direction by the shape
> > of the sail.  Since the air has mass & Newton's
> Laws state that it doesn't
> > "want" to change direction, forcing the airflow to
> change direction requires
> > that work must be done.  The only source of energy
> available to do this work
> > is the kinetic energy of the moving air itself, so
> that's where it must come
> > from.  Macroscopically, we observe this work as an
> increase in the air
> > pressure on the windward side & a decrease in
> pressure on the leeward side
> > of the sail.  The speed of the windward side &
> leeward side airflows adjust
> > themselves in response to these new pressures.
> > 
> > So, what the heck is stalling out?  Well, back to
> Newton's Laws again.
> > Remember the fluid flow does not want to change
> direction.  Forcing the
> > fluid to change direction too abruptly will cause
> the more or less orderly
> > flow of molecules to break down into a more
> chaotic pattern.  The fluid
> > molecules sort of get in each other's way when
> they are forced to change
> > direction too abruptly & go bouncing off in random
> directions.  This process
> > turns the kinetic energy of the fluid flow into
> random molecular vibrations
> > or heat.  We call this process "turbulence". 
> Bernoulli's equation doesn't
> > "care" what form of energy we convert the fluid's
> kinetic energy into, heat
> > is just as good as pressure.  So, at the onset of
> turbulence or stalling,
> > the pressure difference across the sail goes away
> in favor of a slight
> > temperature increase in the airflow.  Again, this
> has been verified
> > experimentally.  Around the turn of the 20th
> century, a British physicist
> > named Osborne Reynolds came up with the concept of
> a dimensionless parameter
> > which could be used to predict the onset of
> turbulence under any set of
> > fluid conditions.  This dimensionless parameter is
> now called the "Reynold's
> > Number" in his honor.  (NOTE: In engineering, one
> of the highest honors is
> > to have a dimensionless number or fundamental
> defining equation named after
> > you!)  The Reynold's Number is given by:
> > 
> > Re = (L * V * ro) / mu
> > 
> > Where:
> > Re = Reynold's Number
> > L = Characteristic Dimension Or Length Of The
> Flowing System (ft)
> > V = Fluid Velocity (ft/sec)
> > ro = Fluid Density (lb/ft^3)
> > mu = Fluid Viscosity (lb/ft-sec)
> > 
> > Note: all the physical parameters that go into
> this calculation must be in
> 
=== message truncated ===



		
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